1. Field of the Invention
The present invention relates to an image encoding apparatus for progressively encoding binary image.
2. Related Background Art
In a facsimile apparatus as a typical still image communication apparatus, a method of encoding an image in accordance with MH or MR coding by sequentially scanning the image is employed. In this method, in order to recognize the entire image, all the image data must be transmitted, resulting in a long transmission time. Therefore, it is difficult to apply this method to an image communication service such as an image data base service, a videotex, or the like, in which an image must be quickly judged.
Unlike the method employed in the facsimile apparatus, a sequential reproducing/encoding method, e.g., progressive coding, is proposed in which rough image data is transmitted before the entire image is transmitted, and additional data is then transmitted to generate detailed image data.
In order to progressively encode binary image data, an image is sub-sampled to generate images having different resolutions, and the images are encoded from those having lower resolutions.
However, if image data is merely sub-sampled to form a low-resolution image, characters or fine lines may be disconnected or omitted before encoding. Therefore, encoded data obtained by encoding the low-resolution image does not properly express an original image. Therefore, an image obtained by decoding such encoded data cannot accurately reproduce the original image.